The present invention relates to angular rate sensors, and more particularly to apparatus and techniques for preventing frequency coupling in devices such as ring laser gyroscopes which compare the frequencies of two counter rotating beams of light as a measure of rotation. Although the present invention is described with particular reference to a ring laser gyroscope, it should be clear that the invention is equally applicable to rotation measuring devices which compare frequencies of other types of electromagnetic radiation to measure rotations.
Ring laser gyroscopes are illustrated and described in U.S. Pat. Nos. 3,390,606, issued in the name of Theodore J. Podgorski and 3,373,650 and 3,467,472, issued in the name of Joseph E. Kilpatrick. The ring laser gyroscopes shown and described in these patents include a triangular block which forms a triangular-shaped ring laser cavity defined by three-corner mirrors. The cavity is filled by a gas which comprises, for example, helium and neon gas.
Two monochromatic laser beams are generated in opposite directions (clockwise and counterwise) around a closed loop optical path in the triangular cavity. Angular movement of the gyroscope in either direction about its axis causes the effective path length for the two beams to change, increasing the path for the beam traveling in the direction of such angular movement and correspondingly decreasing the path of the beam traveling in the opposite direction. Because the closed optical path is a resonant cavity providing sustained oscillation, the wavelength of each beam will also increase or decrease accordingly. Angular rotation of the ring laser gyroscope in either direction about its axis, therefore, causes a frequency differential to occur between the two beam frequencies, which differential is proportional to the rate of angular rotation.
In accordance with the prior art practice, the two beams are extracted from the laser cavity at its output mirror and are heterodyned in a beam combiner to produce an interference pattern. The interference pattern is detected by a photodetector which senses the beat frequency of the heterodyned optical frequencies of the two beams. This beat frequency is a measure of the angular rotation.
However, difficulty arises in ring laser gyroscopes at low rates of rotation. At low rates of rotation, the frequency differential between the two beams is relatively small and the beams tend to couple or resonate together, a phenomenon commonly known as "lock-in", so that the two beams oscillate at only one frequency. Because the frequency differential proportional to the rate of angular rotation no longer exists under such circumstances accurate readings at low rates are difficult to achieve. The lock-in effect is thought to be caused mainly by scattering of light on the mirrors of the gyroscope.
The prior art reveals several approaches for obviating lock-in at low rates of angular rotation. A technique is described in U.S. Pat. Nos. 3,373,650 and 3,467,472 wherein the gyroscope is subjected to a mechanical bias by vibrating so the gyroscope is operating above the lock-in threshold most of the time. The biasing technique commonly used is rotational sinusoidal dithering of the gyroscope about its axis. This technique does not completely eliminate the effects of lock-in because the rotational velocity goes through zero at the extremities of each oscillation and the gyroscope is, therefore, still subject to lock-in. The result of this lock-in is a phase error every time the rotational velocity goes through zero. This error can be cumulative and lead to larger errors in the final output of the gyroscope. A method presently used to overcome this problem is to randomize the bias through the introduction of random noise and thus prevent cumulative errors. Such a random bias technique is described in U.S. Pat. No. 3,467,472.
However, there are some significant disadvantages to the random bias technique. The phase error, though randomized is still one of the largest sources of error in the gyroscope due to random walk error. Moreover, the introduction of random noise vibration requires optical compensation for this noise, usually achieved by mounting the beam combining optics at a stationary (nonvibrating) base. This results in higher susceptibility of the gyroscope to outside vibrations. Mounting more than one gyroscope on a common base becomes extremely difficult because of vibration interaction problems.